Conventionally, a control device of an internal-combustion engine which carries out feedback control of the air/fuel ratio of the air-fuel mixture supplied to an engine (hereinafter, may be simply referred to as an “air/fuel ratio of an engine”) based on each output value of an upper stream side air/fuel ratio sensor and a lower stream side air/fuel ratio sensor disposed on the upper stream side and the lower stream side, respectively, from a three-way catalyst (hereinafter, may be simply referred to as a “catalyst”) in an exhaust passage of an internal-combustion engine is known widely. In such a control device, the upper stream side air/fuel ratio sensor and the lower stream side air/fuel ratio sensor detect the air/fuel ratio of exhaust gas which is discharged from each cylinder of the internal-combustion engine and passes through an exhaust passage (hereinafter, referred to as a “mixed exhaust gas”), and the amount of air/fuel ratio feedbacks is computed using respective air/fuel ratios which each of these sensors detected. Then, by adjusting the quantities of fuel respectively injected to plural cylinders based on the amount of air/fuel ratio feedbacks, feedback control is carried out so that the air/fuel ratio of an engine may correspond to a target air/fuel ratio.
Moreover, in recent years, alcohol, such as ethanol, may be contained in gasoline supplied to an engine as fuel. For example, as for ethanol-containing gasoline fuel used for what is called FFV (Flexible Fuel Vehicle), “E3” in which ethanol concentration is 3%, “E85” in which the concentration is 85%, “E100” which is 100% ethanol fuel, etc. are mainly known, and thus ethanol concentration varies widely. In addition, when alcohol is contained in gasoline like this, such composite fuel of gasoline and alcohol will be referred to as “alcohol-blended fuel” or simply “fuel” hereinafter.
Regarding a control device of an internal-combustion engine which can use such alcohol-blended fuel, for example, the following patent documents 1 (PTL 1) discloses a control device of a flex-fuel internal-combustion engine which changes valve characteristic of an intake valve so that the higher its alcohol concentration is, the faster the flow velocity of the fuel which flows into a combustion chamber through the intake valve by a variable valve actuation mechanism. This conventional control device of a flex-fuel internal-combustion engine is configured so as to shorten the time period during which the fuel contacts the intake valve by increasing the flow velocity of the fuel which flows into the combustion chamber through the intake valve and suppress the evaporation of the fuel due to the heat from the intake valve. Thereby, it is configured so as to inhibit a detergent ingredient added to alcohol-blended fuel from precipitating and depositing on the intake valve.
Moreover, regarding a control device of an internal-combustion engine which can use such alcohol-blended fuel, for example, the following patent documents 2 (PTL 2) discloses a control device of an internal-combustion engine which can start up the engine early even if alcohol-blended fuel which is hard to microatomize under low-temperature environment. This conventional control device of a flex-fuel internal-combustion engine is configured so as to be applicable to an internal-combustion engine equipped with a variable valve timing control device, and it is configured so as to determine the timings for opening and closing an intake valve and/or an exhaust valve based on the alcohol concentration contained in the fuel and microatomize the fuel by gas blown back to an intake passage.
By the way, in an internal-combustion engine which can use alcohol-blended fuel, an intermediate product, such as formaldehyde, may be detected as a detrimental constituent when burning. For this reason, for example, the following patent documents 3 (PTL 3) discloses a valve timing control device of an engine which performs discharge control of formaldehyde accompanying combustion of alcohol-blended fuel by change of valve timing. This conventional control device comprises a valve timing variable mechanism which can change the timings for opening and closing at least one of an intake valve and an exhaust valve, and is configured so as to prolong an overlap period so that combustion temperature may become higher depending on the increase of the detected alcohol concentration in the fuel. Moreover, for example, the following patent documents 4 (PTL 4) discloses a re-burning control device of an engine which suppresses discharge of formaldehyde etc. accompanying combustion of alcohol-blended fuel. This conventional control device is configured so as to detect the operation state of the engine, inhale the burnt gas which was burned in other cylinders depending on the partial load of the engine, and completely burn this inhaled burnt gas.